The key word is “guaranteed”. I go years with the same IP address. But I have no guarantee that it won’t change tomorrow.
The ISP can log which customer has which IP address and when. So if there’s an issue (copyright lawyers, police, etc.) the customer can be tracked down. Having long term static IP addresses makes all this much tidier. You can’t easily claim that you didn’t have that IP address for that day if the ISP logs said you had it for the last 3 years.